A first conventional cathode structure for a color CRT comprises cathodes each comprising an electron emitting layer coated on an outside surface of a closed end of a metal sleeve and a heater for heating the electron emitting layer to a temperature in operation. The cathodes are supported together with a control electrode, a shield electrode, a focusing electrode, and an accelerating electrode by a glass column.
In operation, the electron emitting layers of the cathodes are heated to approximately 800.degree. C. by the heaters, so that electrons are emitted from the electron emitting layers. Beams of the emitted electrons are prevented from the divergence thereof by prefocus lenses defined between the shield electrode and the focusing electrode, and focused on a screen by main lenses defined between the focusing electrode and the accelerating electrode.
In the first conventional cathode structure, however, there are disadvantages in that (1) the time of several seconds takes before an image is displayed on the screen, because the electron emitting layers must be heated to the temperature in operation, (2) heating the electron emitting layers consumes a considerable amount of electric power, and (3) thermal deformation of the electrons must be considered in designing the electrodes, because the electrodes receive heat from the heated cathodes, and the glass column is softened in assembly of the electrodes and the cathodes, and then solidified after assembly thereof.
A second conventional cathode structure for a color CRT which is a field emission type cathode structure is practically used to overcome the disadvantages of the first conventional cathode structure, and comprises cathodes each having a micropoint, a gate electrode having apertures corresponding to the micropoints of the cathodes, and a grid electrode having apertures corresponding to the apertures of the gate electrode. The gate electrode is connected via a bonding wire to a lead connected to a DC power supply.
In operation, a predetermined DC voltage is applied to the gate electrode by the DC power supply, so that an electric field is applied across the micropoints of the cathodes. In this circumstance, when brightness signals of red (R), green (G) and blue (B) are applied to the cathodes, electrons are emitted dependently on the brightness signals from the micropoints of the cathodes to propagate through the apertures of the gate and grid electrodes to the screen.
In the second conventional cathode structure, however, there is disadvantages in that operation of bonding the bonding wire to the gate electrode is difficult to be carried out, because the grid electrode is positioned in the vicinity of the gate electrode by several hundreds to one thousand .mu.m, and a short-circuit tends to occur between the gate and grid electrodes by the bonding wire.